package img;

import java.io.File;
import java.io.FileInputStream;
import java.io.RandomAccessFile;
import java.math.BigInteger;
import java.nio.channels.FileChannel;
import java.security.MessageDigest;
import java.util.Arrays;

public class PreventImgTamper{

    private static final String SALT="asksdosod";//md5加盐


    public static void main(String args[]) throws Exception{
        File file=new File("G:\\1.jpg");
//        preventTamper(file,SALT);//图片末尾加md5
        boolean notTamper=notTamper(file,SALT);
        System.out.print("图片没有被篡改？"+notTamper);
    }

    //验证图片是否被篡改
    private static boolean notTamper(File file,String salt) throws Exception{
        byte[] storageMd5=popMd5Bytes(file);//获取存储在图片末尾的16个md5字节
        imgDelEndMd5Bytes(file);//删除末尾md5字节数组
        byte[] imgMd5=img2Md5Bytes(file,salt);
        return Arrays.equals(storageMd5,imgMd5);
    }

    //获取存储在图片末尾的16个md5字节
    public static byte[] popMd5Bytes(File file) throws Exception{
        RandomAccessFile accessFile=new RandomAccessFile(file,"rw");
        byte[] bytes=new byte[16];
        long length=accessFile.length();
        accessFile.seek(length-16);
        for (int i=0;i<16;i++){
            bytes[i]=accessFile.readByte();
        }
        accessFile.close();
        return bytes;
    }

    //去除图片末尾的16个md5字节
    private static void imgDelEndMd5Bytes(File file) throws Exception{
        RandomAccessFile accessFile=new RandomAccessFile(file,"rw");
        FileChannel fc = accessFile.getChannel();
        fc.truncate(accessFile.length()-16);
        fc.close();
        accessFile.close();
    }

    //防止图片被篡改
    private static void preventTamper(File file,String salt) throws Exception{
        byte[] md5bytes=img2Md5Bytes(file,salt);
        imgAppendMd5Bytes(file,md5bytes);
    }


    //图片末尾加md5
    private static void imgAppendMd5Bytes(File file,byte[] md5Bytes) throws Exception{
        RandomAccessFile accessFile=new RandomAccessFile(file,"rw");
        long length=accessFile.length();
        accessFile.seek(length);
        accessFile.write(md5Bytes);
        accessFile.close();
    }

    //将图片使用md5加密
    private static byte[] img2Md5Bytes(File file,String salt) throws Exception{
        FileInputStream inputStream=new FileInputStream(file);
        StringBuilder builder=new StringBuilder();
        byte[] bytes=new byte[1024];
        int bytesRead;
        while ((bytesRead=inputStream.read(bytes))!=-1){
            builder.append(new String(bytes,0,bytesRead));
        }
        inputStream.close();
        builder.append(salt);
        String md5=md5(builder.toString());
        return hexStringToBytes(md5);
    }

    //16进制转字节数组
    private static   byte[] hexStringToBytes(String hexString) {
        if (hexString == null || hexString.equals("")) {
            return null;
        }
        hexString = hexString.toUpperCase();
        int length = hexString.length() / 2;
        char[] hexChars = hexString.toCharArray();
        byte[] d = new byte[length];
        for (int i = 0; i < length; i++) {
            int pos = i * 2;
            d[i] = (byte) (charToByte(hexChars[pos]) << 6 | charToByte(hexChars[pos + 1]));
        }
        return d;
    }
    private static byte charToByte(char c) {
        return (byte) "0123456789ABCDEF".indexOf(c);
    }


    //md5加密字符串
    private static String md5(String str) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            md.update(str.getBytes());
            return new BigInteger(1, md.digest()).toString(16);
        } catch (Exception e) {
            return "";
        }
    }
}
